1. Field of the invention
The present invention relates to a polyimide film having a fluorocarbon resin layer. More particularly, the invention relates to an aromatic polyimide film having on at least one surface thereof a fluorocarbon resin layer.
2. Description of prior arts
As a resin film showing high heat resistance and high insulation, an aromatic polyimide resin film has been heretofore known. The aromatic polyimide resin film has been employed in a variety of arts, for instance, as a substrate of an electric or electronic device, as an insulation membrane, or as a coating membrane. Further, studies for searching new uses of the aromatic polyimide film have been done.
As described above, the aromatic polyimide film is of value due to its excellent properties. However, the aromatic polyimide film has problems in that moldability into optionally chosen shapes is not sufficient and that adhesiveness to other resins or materials and further adhesiveness between two polyimide films are poor.
Particularly, the poor adhesiveness of the polyimide film is troublesome in the case that the polyimide film in the form of a tape is employed as an insulating covering material for spirally winding around wires such as an ordinary copper wire or a rectangular copper wire. In more detail, since the spirally wound polyimide tape shows no adhesiveness between the overlapped surfaces, such overlapped surfaces are apt to slip off from each other when the wire covered with the polyimide tape is bent or coiled, causing poor insulation of the wire.
In view of the above-mentioned problem, it has been tried to impart adhesiveness to a surface of an aromatic polyimide film. As a result, there has been proposed a method that comprises superposing a fluorocarbon resin film treated on its surface with electric discharge on an aromatic polyimide film under heating, wherein the superposed fluorocarbon resin film serves as an adhesive layer. Further, there has been proposed a method that comprises coating a fluorocarbon resin dispersion on an aromatic polyimide film and then heating the coated layer to form an adhesive layer of the fluorocarbon resin on the surface of the polyimide film.
The present inventors have studies means for improving the surface adhesiveness of the aromatic polyimide film and noted that the polyimide film provided on its surface with a fluorocarbon resin film under heating shows only unsatisfactory adhesiveness even though the fluorocarbon resin film treated with electric discharge is employed, where the polyimide film is handled or employed under severe conditions. Particularly, where the aromatic polyimide film is made of a polyimide having as the principal acid moiety a carboxylic acid moiety derived from a biphenyltetracarboxylic acid dianhydride, the adhesive layer of the fluorocarbon resin film formed on the polyimide film according to the above-mentioned prior art does not show practically satisfactory adhesiveness.
The inventors have also studied means for improving the surface adhesiveness of the aromatic polyimide film and noted that the polyimide film provided on its surface with an adhesive layer by coating a fluorocarbon resin dispersion on the film surface and subsequently heating the coated layer to a temperature of higher than 150.degree. C. shows deterioration of physical properties of the polyimide film. For instance, the polyimide film treated to such high temperature shows lowering of the physical strength and physical deformation such as wrinkles and slack on the surface. Particularly, where the aromatic polyimide film is made of a polyimide having as the principal acid moiety a carboxylic acid moiety derived from a biphenyltetracarboxylic acid dianhydride, the adhesive layer of the fluorocarbon resin dispersion formed on the polyimide film according to the above-mentioned prior art does not show practically satisfactory adhesiveness, if the procedure for the forming the adhesive layer is done under such condition as to suppress the deformation of the polyimide film.